Precision CNC Machining Quotation Guide

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  What a quotation is actually made of
• 2
  Tolerance does not scale linearly with cost
• 3
  Geometry decisions that move the number most
• 4
  Quantity, lead time and the risk premium
• 5
  How to read a quote you did not write
• 6
  Which lever moves the price, and by how much

A CNC machining quotation guide is mostly a list of time and risk. The price you receive is the sum of machine time, setup, material, finishing, inspection and the margin a shop needs to absorb scrap. Nothing else goes in. When two shops differ by 40 percent on the same drawing, the gap is almost always one of those numbers being estimated differently, not one shop being greedy.

Machine time is quoted by the hour, but the useful number is cycle time per part. A 3-axis operation that runs 9 minutes on a 500 × 500 × 450 mm machine costs less per part than a 5-axis cycle running 40 minutes on a machine with a Ø400 mm rotary table. Same drawing, different process plan, different price.

Setup is amortized across the batch. One hour of setup spread over 2 parts adds 30 minutes of billable time to each. Spread over 200 parts it adds 18 seconds. This is the single reason a prototype quote and a production quote for the same geometry look unrelated.

Material is quoted at cut size plus waste, not at finished mass. A bracket that weighs 180 g may start as a 1.2 kg block because the vise needs something to hold. Shops that quote by finished weight are guessing; shops that quote by stock size are reading the drawing.

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  Cycle timeMinutes per part on the specific machine assigned
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  SetupOne-time, divided by batch quantity
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  MaterialStock envelope plus saw kerf and scrap allowance
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  RiskTight tolerance, hard alloy, thin walls, one-off geometry

Most engineers assume half the tolerance means double the price. In practice the curve is flat until it is not. Going from ±0.1 mm to ±0.05 mm on a milled pocket is often free, because the machine holds it without thinking. Going from ±0.02 mm to ±0.005 mm changes the process: temperature-controlled room, lighter finishing passes, more frequent probing, and a CMM report.

The break point sits around ±0.01 mm on aluminum and ±0.005 mm on steel. Below that, you are not buying a better cut. You are buying measurement, documentation and the probability that a rework loop will be needed. That is where a quote jumps.

Surface finish behaves the same way. As-machined at Ra 1.6–3.2 μm comes straight off a sharp cutter. Ra 0.8–1.6 μm needs a finishing pass with reduced feed. Ra 0.2–0.8 μm usually means a second operation, sometimes hand polishing, and it can double the finishing line item on a small part.

One practical rule: only tighten the tolerance on the features that mate with something. A bolt hole pattern at ±0.05 mm is fine. A bearing bore at ±0.005 mm is not optional. Marking the whole drawing at the tightest value is the most expensive habit in the industry.

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  ±0.1 to ±0.05 mmStandard milling and turning, no special controls
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  ±0.05 to ±0.01 mmFinishing passes, in-process probing, stable stock
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  ±0.005 mmTemperature control, CMM report, higher scrap risk

Five sides of work usually need either a 5-axis machine or three separate setups with fixtures. On a part with true position requirements across faces, the 5-axis route wins: one setup, no re-alignment error between operations, shorter total cycle. On a simple rectangular block with holes on two faces, three setups on a 3-axis machine are cheaper because the fixtures already exist.

Deep pockets and tall thin walls raise cost through tool deflection, not through programming. A 4 mm wide pocket at 30 mm deep needs a small-diameter tool with a long reach. You must slow the feed to keep it from chattering. Cycle time can triple compared with a pocket of the same volume at 10 mm deep.

Part size matters less than how it is held. The maximum processing size here is 4,000 mm, and the large machine travel is 4,000 × 400 × 150 mm. A long, slender part at that length may still be cheaper to machine in one pass than a compact part with awkward undercuts, because the long part sits flat on the table and the compact one needs a custom fixture.

Undercuts and internal corners need either a 5-axis approach or an electrode. If the feature is not functional, changing a sharp internal corner to a radius equal to the cutter radius removes an entire operation. Designers who do this routinely cut 15 to 25 percent off a quote.

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  Number of setupsEach re-fixturing adds alignment time and error risk
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  Tool reachDepth-to-diameter above 4:1 slows feeds
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  Internal cornersRadius equal to cutter radius avoids a second op

Setup dominates small quantities. At 1 to 10 parts, roughly half the invoice can be setup and programming. At 200 parts the same setup is a rounding error, and material and cycle time take over. This is why asking for a price break at 50 units often gets a real answer and asking at 5 does not.

Lead time carries a premium that engineers rarely see on the quote. A shop with open capacity quotes normal rates. A shop that must interrupt another job, run a night shift or air-freight material adds a percentage. That percentage is not a fee for speed; it is the cost of the disruption plus the chance the shop is wrong about how fast it can go.

There is no minimum order quantity here, and work runs from one prototype to 10,000+ part runs. Production can start within 24 hours of a released order, and parts typically ship in 3–5 days. Those numbers hold because the shop schedules around the part, not the other way round.

Historical late-delivery probability sits below 2 percent. That figure is not a promise for a specific order. It is context: a very short lead time from a shop with a poor record is worth less than a slightly longer one from a shop that ships when it says.

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  Small batchSetup and programming dominate the invoice
• 2
  Large batchMaterial yield and cycle time dominate
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  Rush orderAdds disruption cost, not machine capability

Ask for the process plan, not just the total. A quote that lists operations, machine type, estimated cycle time and finish specification can be checked against your drawing. A single lump sum cannot. If a supplier will not share the breakdown, you are buying a number, not a service.

Compare quotes line by line against the same process plan. If one quote is much lower, find out where. Usually it is one of four things: a smaller stock envelope, a looser inspection regime, a longer lead time, or an assumption about the drawing that is simply wrong. The last one is the dangerous one, because it surfaces as a non-conforming part.

Check the certification path before you award. ISO 9001:2015 covers general quality systems. IATF 16949:2016 applies to automotive work. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters if you are sending controlled drawings. A quote from a shop without the relevant certificate is not necessarily bad, but you must decide whether you can carry that risk.

Finally, read the assumptions section. Material grade, tolerance default, finish default, inspection level and whether tooling is included. Most disputes trace back to a default nobody read. We run 100 percent inspection before shipment, with raw material checks, in-process monitoring and final inspection reports on request. Those are the things to confirm in writing.

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  Process planOperations, machine type, cycle estimate
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  AssumptionsDefault tolerance, finish, material grade
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  CertificatesMatch the certificate to the end-use industry
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  InspectionDefine what is measured and what is reported